Fluorocarbon-based refrigerants have found widespread use in industry as working fluids for refrigeration, air conditioning and heat pump applications.
Vapor compression is one form of refrigeration. In its simplest form, vapor compression refrigeration involves repetitively vaporizing a liquid refrigerant through absorption of heat at a low pressure and temperature and recondensing the vapor to the liquid phase though heat removal at elevated pressure and temperature. The liquid refrigerant is first vaporized in an evaporator in thermal contact with the mass to be cooled. Heat flowing from the mass to the refrigerant vaporizes the refrigerant. The vapor formed is then removed by means of a compressor, in order to maintain the low pressure in the evaporator. Mechanical energy imparted by the compressor raises the temperature and pressure of the vapor. The high pressure vapor then passes to a condenser for heat exchange with a cooler medium. The cooler medium removes the sensible and latent heats from the vapor thereby condensing the vapor. The liquid refrigerant then passes to an expansion valve and thence to the evaporator for repetition of the cycle.
The choice of the working fluid for any refrigeration system is based on a number of engineering criteria. Typically the refrigerant is chosen so that its normal boiling point (that is, its boiling point at one atmosphere) is below the temperature of the mass to be cooled, so that the vapor pressure of the working fluid is above atmospheric; under these circumstances, leaks do not cause contaminants in the vicinity of the equipment to be drawn into the moving parts of the machinery. The vapor pressure of the working fluid (and hence the gas phase density) also influences the thermodynamic efficiency of the machinery.
In a conventional chilled water air conditioning system, for example, a working fluid is repetitively compressed and expanded as above; the reduced temperature expanded vapor is passed through a heat exchanger comprising a number of tubes containing water, chilling the water. The water is then pumped to areas requiring cooling, where warm air is blown over heat exchange surfaces in thermal relation with the chilled water, chilling the air. Such systems are more economical to operate than systems moving cold air a long distance from a chilling plant to a location to be cooled. Such chilled water refrigeration systems are in common use in buildings, and in ocean-going vessels and the like. A key characteristic of a refrigerant for a chilled water system is that its boiling point be just above 0.degree. C., to avoid freeze-up of the water while obtaining efficient operation.
The majority of refrigerants utilized in vapor compression systems are either single component fluids or azeotropic mixtures. Single component fluids or azeotropic mixtures are characterized as constant boiling because they exhibit isothermal and isobaric evaporation and condensation. Azeotropic or, more generally, azeotrope-like compositions are desired refrigerants because they do not fractionate upon boiling or evaporation. This behavior is desirable because unless the refrigerant is constant-boiling, i.e., is azeotrope-like, fractionation and segregation will occur upon evaporation and condensation carried out in vapor compression equipment. Moreover, leakage from the machinery will cause the composition to shift, altering the properties of the working fluid.
The art is continually seeking new fluorocarbon-based azeotrope-like mixtures for refrigeration and heat pump applications. Environmentally acceptable fluorocarbon-based refrigerants are of particular interest as substitutes for fully halogenated chlorofluorocarbons (CFCs) which have been for many years the dominant refrigerants. CFCs are implicated in causing environmental problems associated with the depletion of the earth's protective ozone layer.
Any CFC substitute material must possess the useful properties of the CFC to be replaced, including chemical stability, low toxicity, non-flammability, and efficiency in use. The latter characteristic is important, for example, in refrigeration applications where a loss in refrigerant thermodynamic performance or energy efficiency may produce secondary environmental effects due to increased fossil fuel usage arising from an increased demand for electrical energy. Furthermore, an ideal CFC substitute refrigerant would not require major engineering changes to vapor compression technology currently used with CFC refrigerants. Thus, a CFC substitute must have boiling point, speed of sound, thermal conductivity and like characteristics similar to the CFC it is to replace.
The present invention concerns replacement of CFC refrigerant fluids used in water chillers specifically intended for marine application, that is, where the system may at some time be exposed to salt water. This factor presents an additional constraint on the selection of a refrigerant; specifically, one must take into account the possibility that at some point the components of the system may leak. If the vapor pressure of the working fluid is below atmospheric at the usual working temperature of the system, such a leak may cause water or water vapor in the vicinity of the leak to be drawn into the system. If salt water is drawn into a typical vapor compressor the compressor may be very badly damaged and possibly destroyed.
Refrigerant fluids for use in chilled water refrigeration units for marine application have been selected to have above-atmospheric vapor pressures at the usual working temperature, so that a leak does not tend to draw salt water into the components in contact with the working fluid. Specifically, a known refrigerant referred to in the industry as R114, that is, CClF.sub.2 CClF.sub.2, has been widely used in water chillers for marine use. This compound is a fully halogenated chlorofluorocarbon (CFC). Such CFC's are rapidly being eliminated throughout industry due to their tendency to cause environmental damage, specifically to the ozone layer of the atmosphere. Therefore a need exists in the art for a replacement for R114, retaining its desirable physical properties, but avoiding chlorine or other halogens as constituents.
Fluoro-ethers, such as pentafluorodimethyl ether, are known as potentially useful refrigerants. See U.S. Pat. Nos. 3,362,180 and 3,922,228. In particular, bis-(difluoromethyl)ether, CHF.sub.2 OCHF.sub.2, has been suggested for use as a refrigerant, either alone or in combination with other refrigerants. See U.S. Pat. No. 4,961,321 to O'Neill et al. However, no particular examples of suitable combinations are given. The O'Neill patent discloses that bis-(difluoromethyl)ether is physically similar to the R114 refrigerant used in marine water chilling systems.
U.S. Pat. No. 4,948,526 to Fellows et al discusses combinations of pentafluorodimethyl ether and monochloro-difluoromethane for use in heating and cooling applications, that is, as refrigerants for heat pumps.
U.S. Pat. No. 4,810,403 to Bivens et al discusses a variety of halocarbon blends for refrigerant use, specifically including FC-143 (also known in the art as R143), that is, CHF.sub.2 CH.sub.2 F, or 1,1,2-trifluoroethane. However, the combinations disclosed in the Bivens et al patent do not suggest combination of FC-143 or other halocarbons with ethers. Moreover, the data given by Bivens et al regarding FC-143 is not accurate, according to detailed measurements carried out by the present inventor; in general, FC-143 has not been well characterized in the literature and has not been the subject of detailed study.
U.S. Pat. No. 2,066,905 to Booth discusses halogenated methyl ethers generally for use as refrigerants.
U.S. Pat. No. 4,139,607 to Simons et al discusses fluorinated dimethyl ethers as aerosol propellants, including bis-(difluoromethyl)ether.
U.S. Pat. No. 4,041,148 also to Simons et al is generally similar.
Accordingly, it is an object of this invention to provide azeotrope-like compositions for use as CFC substitute refrigerants, avoiding the use of chlorinated fluorocarbons.
It is another object of the invention to provide an environmentally acceptable, chemically stable, non-flammable and energy efficient refrigerant suitable for use in water chillers, specifically marine water chillers.